Mobile communications systems, such as UMTS, typically comprise data channels for the transmission of data packets and control channels for the transmission of control information. The data channels and control channels are transmitted in time slots of predetermined duration.
By way of example, in UMTS, uplink data is transmitted using the Enhanced Dedicated Channel (E-DCH) and downlink data is transmitted using the High-Speed Downlink Shared Channel (HS-DSCH). These data channels are designed to be highly variable in data rate and to be not transmitted when there is no data to transmit. In parallel with these data channels, control channels are also transmitted. For example, in UMTS three uplink control channels are used, the DPCCH (Dedicated Physical Control Channel), the HS-DPCCH (High Speed Dedicated Physical Control Channel) and the E-DPCCH (Enhanced Dedicated Physical Control Channel).
The E-DPCCH primarily carries information to enable the Base Station (BS) to decode the E-DCH data, and is only transmitted when E-DCH data is being transmitted. The HS-DPCCH carries data such as automatic repeat request (ARQ) acknowledgements for the downlink HS-DSCH data and periodic downlink channel quality indicators (CQI); the HS-DPCCH is usually not transmitted unless positive or negative acknowledgements (ACK/NACK) or CQI need to be transmitted.
The DPCCH carries at least pilot bits and power control commands. The pilot bits are of predetermined value and provide a phase reference for the decoding of other uplink channels and DPCCH fields, and may be used for channel estimation. The pilot bits may also be used by the BS to measure the uplink signal-to-interference ratio (SIR) for the purpose of generating the power control commands that are transmitted on the downlink.
In many packet data applications, such as web browsing and email, the data transmission is bursty, with periods of high data rates interspersed with “reading periods” when no data is transmitted. During periods when no data is transmitted, it is advantageous to minimise the overheads of running the control channels in order to reduce interference to other users. However, for a good user experience, it is also necessary to minimise delays at the start of each burst of data packets. For this reason the uplink and downlink DPCCHs are typically kept running to maintain the closed-loop power control and avoid lengthy set-up delays and power control convergence periods.
One way, included in Release 6 of the UMT specifications, of reducing the downlink control channel overheads is to employ a Fractional Dedicated Channel (F-DPCH), which comprises only power control commands and therefore enables multiple users' control channels to be multiplexed together in a time-division manner within timeslots. This is possible for the downlink as a common pilot channel (CPICH) exists which can be used as the phase reference for the power control commands. For the uplink, however, no other phase reference is available than the pilot bits provided by the DPCCH, so it is necessary to continue to transmit both phase reference and power control information.
In a communication system, and in CDMA communication systems in particular, it is generally desirable to minimise the amount of transmission, and the transmission power level, in order to minimise the possibility of causing interference to other users of the system, and hence to maximise the system capacity for other users. Minimising the amount of transmitted energy also has the advantage of increasing the battery life of mobile terminals.
One problem with the uplink DPCCH is that the pilot field has to provide the channel estimate for decoding data channels when data is transmitted. However, this means that the amount of pilot energy is excessive for the periods between data transmission, when only power control commands have to be decoded.
One possible technique is to reduce the transmission power level of the DPCCH when data is not being transmitted. This power reduction may be applied to the whole DPCCH, or just to the pilot field. If it is applied to the whole DPCCH, it will affect the reliability of the power control commands, either making them unnecessarily reliable when data is transmitted or making them too unreliable when data is not transmitted. If the power reduction is applied only to the pilot field, the resulting transmission power of the DPCCH will be non-uniform, which is undesirable because it is then necessary for the transmitter to adjust the transmit power more than once per timeslot, thus increasing the complexity of the device.